This invention relates to circuit breakers of the molded case, narrow width type such as is shown and described in U.S. Pat. No. 3,081,386 to M. F. Koenig, et al, dated Mar. 12, 1963 and owned by mesne assignments by the assignee herein. Circuit breakers of this type are normally utilized in residential and commercial applications up to 240 volts.
Circuit breakers of the above type commonly employ spring loaded latch trip mechanisms which are manually operable to selectively open and close the circuit breaker contacts and which are automatically operable in response to overload current conditions to separate the contacts. Automatic operation occurs when a magnetic or thermal responsive sensor operates to unlatch the mechanism in response to overload currents. In the interest of effective space utilization the magnetic and thermal responsive sensors are commonly interrelated. For example, the thermal sensor may be a bimetal member which also serves as a single turn winding for an electromagnetic core of the magnetic sensor. The latter has a movable armature associated therewith which in turn has a latch trip catch member for releasing the spring loaded latch trip operating mechanism upon attraction of the armature to the core. A lost motion mechanical connection between the bimetal and the movable armature permits the bimetal member to operate independently of the magnetic sensor to trip the latched operating mechanism. Small overload currents are detected by the bimetal thermal sensor, causing it to deflect and pull the armature to a latch releasing position. Higher fault current conditions are detected by the magnetic sensor wherein the current passing through the bimetal member generates an electromagnetic field within the core to attract the armature and release the latch before the bimetal warps to its deflected position due to the same overload currents. Thus the magnetic sensor responds more quickly to higher overload current conditions than does the thermal sensing bimetal, but each operate to separate the contacts by unlatching the operating mechanism to cause it to move to a tripped position and thereby open the contacts.
The amount of current being made available by electrical utilities to the residential or commercial customer is increasing in newer installations. Accordingly, the short circuit currents available in these newer installations increases significantly and the interrupting capability of circuit breakers must also increase. One method of increasing the interrupting capability of an electric circuit breaker is to increase the speed with which the circuit breaker operates to separate the contacts in response to fault current conditions. Circuit breakers which employ the spring loaded operating mechanism in conjunction with a releasable latch mechanism for the magnetic trip function are limited in their contact separation speed because the operating mechanism moves sequentially after release of the latch by the magnetic sensor to cause the mechanism to toggle the contacts open.